1. Field of the Invention (Technical Field)
The present invention relates to methods of analyzing trace constituents present in inert gases and, more particularly, the measurement of trace analytes through the use of a dielectric barrier discharge. Still more particularly, it relates to the analysis of trace constituents which have become excited by their collision with an excited inert gas, the inert gas having been elevated to an excited state by its passage through a dielectric barrier discharge. The dielectric barrier discharge is formed from the application of high voltage alternating current to two electrodes placed in opposition separated by a dielectric material.
2. Background Art
The need to measure trace analytes is nearly the raison d'etre for analytical chemistry. Whether the analytes are impurities which may affect an etching process in a semiconductor plant or they are trace levels of health-affecting pesticides in drinking water, the need to measure these analytes at low levels is very important. In view of these needs, new methods of determining trace level analytes are of growing importance.
Many different techniques have been identified for the measurement of trace analytes in inert gases. Capelle et al., U.S. Pat. No. 4,150,951, Taylor et al., U.S. Pat. No. 4,148,612, Ault, U.S. Pat. No. 3,545,863, and Dodge, III et al., U.S. Pat. No. 4,309,187, all disclose methods of spectroscopically measuring trace analytes in an inert gas through the use of various excitation sources. Dodge, III et al., details a process where a dielectric barrier discharge at low pressure is utilized to form metastable states of nitrogen which are then used to spectroscopically determine various metallic analytes such as zinc and mercury by their respective emission lines. Monagle et al., U.S. patent application Ser. No. 08/516,838, discloses a process wherein a dielectric barrier discharge plasma is used to oxidize halogenated hydrocarbons into their respective acid species which are subsequently measured. Scott et al., U.S. Pat. No. 4,873,862 and Simpson, U.S. Pat. No. 4,740,695, both disclose mechanisms for generating ionization within argon. Scott utilizes a thermal effect while Simpson discloses a means of ionizing argon using an ultraviolet source and a photoemissive element. These ionization processes are described as a means of creating metastable state species in argon in order to selectively ionize other constituents of interest. Stearns et al., U.S. Pat. Nos. 5,394,091 and 5,317,271, disclose a method of analyzing trace species in a gas stream using the ionization of the species through a charge transfer mechanism after it has come in contact with helium that has been put into an excited state. This excited state is obtained by a controlled, direct current (DC), pulsed spark discharge across two metal electrodes. The discharge is generated for a discrete time period as controlled by the circuitry identified in the patents identified. The balance of the time is spent with the discharge off, a state in which no additional excitation or ionization is taking place directly in the electrode path.
The methods identified above as well as the commercially available methods of doing the same have several drawbacks. To measure emission lines requires the support of a photomultiplier tube. Photomultiplier tubes yield the lowest noise at lower temperatures but chromatographic applications typically require the use of high temperatures to prevent analyte condensation. The methods identified by Simpson and Scott et al. will only yield limited quantities of ionized species which limits the linear range of detection. The use of exposed electrodes, as described by Wentworth et al. exposes the electrodes to contamination which will limit the useful operating life-time of the discharge system. Flame ionization based mechanisms, while they have been in use for many years, require several supporting gases and can, by their very nature, provide an ignition safety concern.
Accordingly, the need exists for a method of detecting trace constituents sensitively, with a large linear dynamic range, and with a simple and convenient apparatus. An additional capability to selectively identify materials is further desired.